1. Field of the Invention
The present invention relates to a light pickup device and an optical disk apparatus. Particularly, the invention relates to a light pickup device and an optical disk apparatus for high density optical disks which require a high-quality light spot, and to a light pickup device and an optical disk apparatus which employ a light detector including a plurality of light receiving members for servo control.
2. Description of the Related Art
Japanese Unexamined Patent Publication No. Sho 63-167438 (1988), for example, discloses a prior-art optical disk apparatus related to the present invention, which comprises: an optical disk as an information recording medium; a semiconductor laser as a light source; a collimator lens which collimates light emitted from the semiconductor laser into a parallel light beam; a mirror which reflects the parallel light beam; beam shaping means which receives the light beam reflected from the mirror and shapes the light beam in a generally round cross section; an objective lens which receives the light beam shaped by the beam shaping means and forms a light spot on the optical disk; focusing error detection means which receives light reflected from the optical disk and detects a signal; tracking error detection means; information detection means; and an optical block which fixedly holds all the components; wherein the mirror is capable of performing at least one of operations for changing the angle of the axis of the light beam reflected from the mirror with respect to the optical axis of the mirror about a reflection point and for moving the axis of the reflected light in parallel without changing the angle of the reflected light axis so as to coincide the axis of the light beam incident on the objective lens with the optical axis of the objective lens. An optical disk apparatus including a light pickup device having a construction as shown in FIG. 23 is also known.
In the apparatus shown in FIG. 23, light emitted from a light source 101 passes through a half mirror 106 (beam splitter) and a collimator lens 105, and is condensed on an optical disk 103 by an objective lens 104. Then, the light reflected from the optical disk 103 passes through the objective lens 104, the collimator lens 105, the half mirror 106 and a cylindrical convex lens 108, and is finally condensed into a light spot on a light detector 102.
A semiconductor laser is typically employed as the light source 101. In general, the center axis of a light intensity distribution of the light emitted from the semiconductor laser is offset from the optical axis of the semiconductor laser by 2 to 3 degrees. Where such a semiconductor laser is mounted on the light pickup device, the center axis of the light intensity distribution of the emitted light is offset from the optical axis 121 by 2 to 3 degrees. This results in deformation of the light spot on the optical disk and variations in light transmission efficiency. However, these drawbacks present no serious problem in practical applications of the light pickup device and the optical disk apparatus for conventional low density optical disks such as CDs.
A light detector comprising a plurality of light receiving members is generally employed for detection of information signals and servo signals from the optical disk. In this case, the amounts of light incident on the respective light receiving members of the light detector are liable to be unbalanced due to the offset of the light intensity center of the semiconductor laser and displacement of the optical components. The unbalanced light amounts disturb the servo signals, but present no serious problem in practical applications of the light pickup device and the optical disk apparatus for the low density optical disks (e.g., CDs).
In the case of high density optical disks such as DVDs, however, proper detection of the information signals is made difficult by the deformation of the light spot on the optical disk due to the offset of the center axis of the light intensity distribution of the semiconductor laser, the variations in the light transmission efficiency, and the disturbance of the servo signals due to the unbalanced light amounts on the respective light receiving members of the light detector.